The present invention relates to an audio power amplifier and, more particularly, to a protection circuit for protecting a switching circuit from possible malfunction or damage due to sudden power source voltage fluctuation.
Class D audio power amplifier is one type of switching circuit. Therefore, Class D audio power amplifier is chosen for illustration purpose. Most of the audio power amplifiers in the market are based on Class AB amplifier. This architecture offers very good total harmonic distortion plus noise (THD+N) performance, with fairly low quiescent current. However, the Class AB push-pull amplifiers are very inefficient and can only achieve an efficiency of about 60%, which results in not only power loss, but also additional bulky heatsink attached to the power amplifiers.
One major advantage of Class D amplifiers is the efficiency, which could reach above 90%. The high efficiency is achieved by full signal swing at power transistors. A typical Class D amplifier circuit 1000 is shown in FIG. 1, which includes a reference (herein after also indicated as RF) circuit 1020, a pulse width modulator 1010, a level shifter and driver stage 1030, a first MOSFET switch M10, and a second MOSFET switch M20.
In the actual usage of Class D amplifier circuit 1000, the power source voltage Vcc may fluctuate suddenly and abnormally. Such sudden voltage fluctuation is due to, for example, transformer malfunction on a printed circuit board (PCB), power supply trace shorting to ground on PCB or other scenarios. If such sudden voltage fluctuation should occur, Class D amplifier circuit 1000 is damaged or performs abnormal operation.
To prevent Class D amplifier circuit 1000 from possible damaging or malfunction due to sudden power source voltage Vcc fluctuation, a protection circuit is necessary.
The following prior art references are known.    1) Japanese Utility Model Laid-Open Publication No. H5-39095 Published May 25, 1993    2) Japanese Patent Laid-Open Publication No. H4-108204 Published Apr. 9, 1992    3) Japanese Patent Laid-Open Publication No. S58-81311 Published May 16, 1983